1. Field of the Invention
The present invention relates to an apparatus for controlling a lock-up clutch of a lock-up type torue converter f an automatic transmission, and more particularly to an apparatus for controlling engagement/disengagement of a lock-up clutch of a lock-up type torque converter of an automatic transmission in response to a change in surface condition of road which an automobile installed with the automatic transmission is running through.
2. Description of Prior Art
The lock-up type torque converters have been developed for the purpose of preventing a drop in power transmission efficiency through the torque converters which would occur during operation at high vehicle speeds. There have been a various kinds of proposal regarding lock-up control.
JP-PS No. 60-2549, which corresponds to U.S. Pat. No. 4,431,095, discloses a lock-up type automatic transmission wherein a lock-up clutch is engaged as long as a vehicle speed attains a predetermined relationship with one of predetermined vehicle speed values which is selected for a gear ratio established although the engagement of the lock-up clutch is temporarily released during shifting operation.
JP-PS No. 60-2550, which corresponds to U.S. Pat. No. 4,499,979, discloses a lock-up control system wherein a similar lock-up control is carried out based on result of comparison of an actual vehicle speed with a predetermined so-called "lock-up vehicle speed," the actual vehicle speed being indicated by content of a counter which counts sensor pulses having a frequency variable with the vehicle speed within a time interval between adjacent two clock pulses.
JP-PS No. 59-43664 discloses a lock-up type automatic transmission wherein engagement of a lock-up clutch is released immediately after a brake pedal has been depressed. This control is intended to prevent stalling of an engine which might be caused if the lock-up clutch were kept engaged.
JP-PS No. 59-43666 discloses a lock-up type automatic transmission wherein engagement of a lock-up clutch is released only when the vehicle is subject to rapid deceleration.
However, the above mentioned various kinds of proposal according to the prior art cannot cope with a change in road surface condition that affects friction coefficient between road surface and road wheels of an automobile.
The road surface condition varies and there may occur stalling of the engine if the road wheels lose traction and lock up during braking operation under conditions where the vehicle is travelling through wet road with the lock-up clutch kept engaged. this is because there is no slip between the engine and the braked wheels and thus the braked wheels bring the engine to a stall when they lock up.
A task of the present invention is to improve a lock-up control such that engagement of a lock-up clutch is prohibited so as to prevent stalling of an engine which otherwise would occur if braked wheels lock up on road surface condition where the wheels tend to lose traction.